Regulation and function of adult neurogenesis: from genes to cognition

Repeated restraint stress promotes hippocampal neuronal cell ciliogenesis and proliferation in mice

Stress severely disturbs physiological and mental homeostasis which includes adult neurogenesis in hippocampus. Neurogenesis in hippocampus is a key feature to adapt to environmental changes and highly regulated by multiple cellular signaling pathways. The primary cilium is a cellular organelle, which acts as a signaling center during development and neurogenesis in adult mice. However, it is not clear how the primary cilia are involved in the process of restraint (RST) stress response. Using a mouse model, we examined the role of primary cilia in repeated and acute RST stress response. Interestingly, RST stress increased the number of ciliated cells in the adult hippocampal dentate gyrus (DG). In our RST model, cell proliferation in the DG also increased in a time-dependent manner. Moreover, the analysis of ciliated cells in the hippocampal DG with cell type markers indicated that cells that were ciliated in response to acute RST stress are neurons. Taken together, these findings suggest that RST stress response is closely associated with an increase in the number of ciliated neurons and leads to an increase in cell proliferation.

Involvement of β-catenin in cytoskeleton disruption following adult neural stem cell exposure to low-level silver nanoparticles

Silver nanoparticles (AgNPs) are increasingly incorporated in consumer products to confer antibacterial properties. AgNPs are shed during everyday use of these products, resulting in ingestion or inhalation and bioaccumulation in tissues including the brain. While these low levels of AgNPs do not induce DNA fragmentation typical of apoptosis or necrosis, they do interfere with cytoskeletal structure and dynamics in cultured differentiating adult neural stem cells (NSCs). Moreover, these cells form f-actin inclusions in response to 1 μg/ml AgNPs. Here, we report that these cytoskeletal inclusions colocalize with aggregates of the signaling protein β-catenin, a modulator of cytoskeletal dynamics. Pharmacological alteration of β-catenin signaling reduced formation of f-actin inclusions. AgNP exposure also resulted in a reduction of neurite length in differentiating NSCs, which was mimicked by pharmacological activation of β-catenin signaling. Conversely, pharmacological inhibition of the Wnt/β-catenin signaling pathway resulted in increased neurite lengths in control cells, but did not reverse the neurite collapse induced by AgNP exposure. Substantial changes in neurite length, in response to low-level AgNP or pharmacological manipulation of β-catenin signaling, occurred within the first six hours of exposure and were most evident in cells differentiating towards neural-like morphologies. We conclude that low-level exposure to AgNP, such as that resulting from use of consumer products, may disrupt β-catenin signaling in neural cells in an indirect or non-additive manner. Exposure to AgNP shed from consumer products at levels currently considered safe, may therefore alter physiological function of neural cells. This is of concern particularly regarding children, whose brains contain many developing neurons, and who may face bioaccumulation of AgNP over decades of exposure.

An Immune-CNS Axis Activates Remote Hippocampal Stem Cells Following Spinal Transection Injury

External stimuli such as injury, learning, or stress influence the production of neurons by neural stem cells (NSCs) in the adult mammalian brain. These external stimuli directly impact stem cell activity by influencing areas directly connected or in close proximity to the neurogenic niches of the adult brain. However, very little is known on how distant injuries affect NSC activation state. In this study, we demonstrate that a thoracic spinal transection injury activates the distally located hippocampal-NSCs. This activation leads to a transient increase production of neurons that functionally integrate to improve animal's performance in hippocampal-related memory tasks. We further show that interferon-CD95 signaling is required to promote injury-mediated activation of remote NSCs. Thus, we identify an immune-CNS axis responsible for injury-mediated activation of remotely located NSCs.

The α7 nicotinic acetylcholine receptors regulate hippocampal adult-neurogenesis in a sexually dimorphic fashion

Disruption in cholinergic signaling has been linked to many environmental and/or pathological conditions known to modify adult neurogenesis. The α7 nAChRs are in the family of cys-loop receptor channels which have been shown to be neuroprotective in adult neurons and are thought to be critical for survival and integration of immature neurons. However, in developing neurons, poor calcium buffering may cause α7 nAChR activation to be neurotoxic. To investigate whether the α7 nAChR regulates neurogenesis in the hippocampus, we used a combination of mouse genetics and imaging to quantify neural stem cell (NSC) densities located in the dentate gyrus of adult mice. In addition, we considered whether the loss of α7 nAChRs had functional consequences on a spatial discrimination task that is thought to rely on pattern separation mechanisms. We found that the loss of α7 nAChRs resulted in increased neurogenesis in male mice only, while female mice showed increased cell divisions and intermediate progenitors but no change in neurogenesis. Knocking out the α7 nAChR from nestin⁺ NSCs and their progeny showed signaling in these cells contributes to regulating neurogenesis. In addition, male, but not female, mice lacking α7 nAChRs performed significantly worse in the spatial discrimination task. This task was sexually dimorphic in wild-type mice, but not in the absence of α7 nAChRs. We conclude that α7 nAChRs regulate adult neurogenesis and impact spatial discrimination function in male, but not female mice, via a mechanism involving nestin⁺ NSCs and their progeny.

Growth Differentiation Factor 11 treatment leads to neuronal and vascular improvements in the hippocampus of aged mice

Aging is the biggest risk factor for several neurodegenerative diseases. Parabiosis experiments have established that old mouse brains are improved by exposure to young mouse blood. Previously, our lab showed that delivery of Growth Differentiation Factor 11 (GDF11) to the bloodstream increases the number of neural stem cells and positively affects vasculature in the subventricular zone of old mice. Our new study demonstrates that GDF11 enhances hippocampal neurogenesis, improves vasculature and increases markers of neuronal activity and plasticity in the hippocampus and cortex of old mice. Our experiments also demonstrate that systemically delivered GDF11, rather than crossing the blood brain barrier, exerts at least some of its effects by acting on brain endothelial cells. Thus, by targeting the cerebral vasculature, GDF11 has a very different mechanism from that of previously studied circulating factors acting to improve central nervous system (CNS) function without entering the CNS.

Age-related changes in Ki-67 and DCX expression in the BALB/ c mouse (Mus Musculus) brain

Several studies have identified age as one of the strongest regulators of neurogenesis in the mammalian brain. However, previous age-related studies focused mainly on changes in neurogenesis during different stages of adulthood and did not describe changes in neurogenesis through the different life history stages of the animal. The aim of this study was therefore to determine time course changes in neurogenesis in the male BALB/c mouse brain at postnatal ages 1 week to 12 weeks, spanning juvenile, sub adult and adult life history stages. To achieve this, Ki-67 and DCX immunohistochemistry was used to assess changes in cell proliferation and neuronal incorporation respectively. Ki-67 expression was mainly observed in the olfactory bulb, rostral migratory stream, sub ventricular zone of lateral ventricle and the sub granular zone of the dentate gyrus. In addition, fewer Ki-67 positive cells were also observed in the neocortex, cerebellum and tectum. DCX was expressed in similar regions as Ki-67 except for the cerebellum and tectum. Expression of both Ki-67 and DCX sharply decreased with advancing age or life history stages in the sub ventricular zone, rostral migratory stream and sub granular zone of the BALB/c mouse brain. Neurogenesis therefore persists throughout all life history stages in the BALB/c mouse brain although it decreases with age.

Human UCB-MSCs treatment upon intraventricular hemorrhage contributes to attenuate hippocampal neuron loss and circuit damage through BDNF-CREB signaling

Background

Human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) have been shown to prevent brain damage and improve neurocognition following intraventricular hemorrhage (IVH). However, the molecular mechanisms underlying the effects of hUCB-MSCs are still elusive. Thus, as the hippocampus is essential for learning, memory, and cognitive functions and is intimately involved in the ventricular system, making it a potential site of IVH-induced injury, we determined the molecular basis of the effects of hUCB-derived MSCs on hippocampal neurogenesis and the recovery of hippocampal neural circuits after IVH in a rodent model.

Methods

We inflicted severe IVH injury on postnatal day 4 (P4) in rats. After confirmation of successful induction of IVH using MRI (P5), intracerebroventricular administration of MSCs (ICV-MSC) was performed at 2 days post-injury (P6). For hippocampal synaptic determination, a rat entorhinal-hippocampus (EH) organotypic slice co-culture (OSC) was performed using day 3 post-IVH brains (P7) with or without ICV-MSCs. A similar strategy of experiments was applied to those rats receiving hUCB-MSC transfected with BDNF-Si-RNA for knockdown of BDNF or scrambled siRNA controls after IVH. The molecular mechanism of the MSCs effects on neurogenesis and the attenuation of neuron death was determined by evaluation of BDNF-TrkB-Akt-CREB signaling axis.

Results

We showed that treatment with hUCB-MSCs attenuated neuronal loss and promoted neurogenesis in the hippocampus, an area highly vulnerable to IVH-induced brain injury. hUCB-MSCs activate BDNF-TrkB receptor signaling, eliciting intracellular activation of Akt and/or Erk and subsequent phosphorylation of CREB, which is responsible for promoting rat BDNF transcription. In addition to the beneficial effects of neuroprotection and neurogenesis, hUCB-MSCs also contribute to the restoration of impaired synaptic circuits in the hippocampus and improve neurocognitive functions in IVH-injured neonatal rat through BDNF-TrkB-CREB signaling axis activation.

Conclusions

Our data suggest that hUCB-MSCs possess therapeutic potential for treating neuronal loss and neurocognitive dysfunction in IVH through the activation of intracellular TrkB-CREB signaling that is invoked by hUCB-MSC-secreted BDNF.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1052-5) contains supplementary material, which is available to authorized users.

Brain Plasticity in Mammals: An Example for the Role of Comparative Medicine in the Neurosciences

Comparative medicine deals with similarities and differences between veterinary and human medicine. All mammals share most basic cellular and molecular mechanisms, thus justifying murine animal models in a translational perspective; yet “mice are not men,” thus some biases can emerge when complex biological processes are concerned. Brain plasticity is a cutting-edge, expanding topic in the field of Neurosciences with important translational implications, yet, with remarkable differences among mammals, as emerging from comparative studies. In particular, adult neurogenesis (the genesis of new neurons from brain stem cell niches) is a life-long process in laboratory rodents but a vestigial, mostly postnatal remnant in humans and dolphins. Another form of “whole cell” plasticity consisting of a population of “immature” neurons which are generated prenatally but continue to express markers of immaturity during adulthood has gained interest more recently, as a reservoir of young neurons in the adult brain. The distribution of the immature neurons also seems quite heterogeneous among different animal species, being confined within the paleocortex in rodents while extending into neocortex in other mammals. A recent study carried out in sheep, definitely showed that gyrencephalic, large-sized brains do host higher amounts of immature neurons, also involving subcortical, white, and gray matter regions. Hence, “whole cell” plasticity such as adult neurogenesis and immature neurons are biological processes which, as a whole, cannot be studied exclusively in laboratory rodents, but require investigation in comparative medicine, involving large-sized, long-living mammals, in order to gain insights for translational purposes.

Neurogenesis within the hippocampus after chronic methylphenidate exposure

Methylphenidate is a psychostimulant used to treat attention deficit hyperactivity disorder. Neurogenesis occurs throughout adulthood within the dentate gyrus of the hippocampus and can be altered by psychoactive medications; however, the impact of methylphenidate on neurogenesis is not fully understood. We investigated the effects of chronic low (1 mg/kg) and high (10 mg/kg) intraperitoneal doses of methylphenidate on neurogenesis in mouse hippocampus following 28 days and 56 days of treatment. Interestingly, methylphenidate, at both doses, increased neurogenesis. However, if methylphenidate treatment was not continued, the newly generated cells did not survive after 28 days. If treatment was continued, the newly generated neurons survived only in the mice receiving low-dose methylphenidate. To investigate the mechanism for this effect, we examined levels of proteins linked to cell proliferation in the hippocampus, including brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), vascular endothelial growth factor (VEGF), tropomyosin receptor kinase B (TrkB), and beta-catenin. BDNF or GDNF levels were not significantly different between groups. However, hippocampal VEGF, TrkB, and beta-catenin were significantly increased in mice receiving low-dose methylphenidate for 28 days compared to controls. Interestingly, high-dose methylphenidate significantly decreased beta-catenin after 28 days and decreased VEGF, beta-catenin, and TrkB after 56 days compared to controls. Thus, low-dose methylphenidate appears to increase cell proliferation and cell survival in the hippocampus, and these effects may be mediated by increase in VEGF, TrkB, and beta-catenin. While high dose methylphenidate may initially increase neuronal proliferation, newly generated neurons are unable to survive long-term, possibly due to decrease in VEGF, TrkB and beta-catenin.

Noggin rescues age-related stem cell loss in the brain of senescent mice with neurodegenerative pathology

Alzheimer’s disease (AD) is the most common cause of age-related neurodegeneration. Damage initially occurs in the hippocampus, a neurogenic brain region essential in forming memories. Since there is no cure for AD, therapeutic strategies that may aid to slow hippocampal dysfunction are necessary. We describe the precocious hippocampal stem cell loss of a mouse model that mimics the onset of pathological AD-like neurodegeneration. The loss is due to an increase in BMP6 that limits neurogenesis. We demonstrate that blocking BMP signaling by means of Noggin administration is beneficial to the hippocampal microenvironment, restoring stem cell numbers, neurogenesis, and behavior. Our findings support further development of BMP antagonists into translatable molecules for the rescue of stem cells and neurogenesis in neurodegeneration/aging.

Increasing age is the greatest known risk factor for the sporadic late-onset forms of neurodegenerative disorders such as Alzheimer’s disease (AD). One of the brain regions most severely affected in AD is the hippocampus, a privileged structure that contains adult neural stem cells (NSCs) with neurogenic capacity. Hippocampal neurogenesis decreases during aging and the decrease is exacerbated in AD, but the mechanistic causes underlying this progressive decline remain largely unexplored. We here investigated the effect of age on NSCs and neurogenesis by analyzing the senescence accelerated mouse prone 8 (SAMP8) strain, a nontransgenic short-lived strain that spontaneously develops a pathological profile similar to that of AD and that has been employed as a model system to study the transition from healthy aging to neurodegeneration. We show that SAMP8 mice display an accelerated loss of the NSC pool that coincides with an aberrant rise in BMP6 protein, enhanced canonical BMP signaling, and increased astroglial differentiation. In vitro assays demonstrate that BMP6 severely impairs NSC expansion and promotes NSC differentiation into postmitotic astrocytes. Blocking the dysregulation of the BMP pathway and its progliogenic effect in vivo by intracranial delivery of the antagonist Noggin restores hippocampal NSC numbers, neurogenesis, and behavior in SAMP8 mice. Thus, manipulating the local microenvironment of the NSC pool counteracts hippocampal dysfunction in pathological aging. Our results shed light on interventions that may allow taking advantage of the brain’s natural plastic capacity to enhance cognitive function in late adulthood and in chronic neurodegenerative diseases such as AD.

Short term exposure to oxycodone alters the survival, proliferation and differentiation of rat embryonic neural stem cell in vitro

As one of several opioids, oxycodone has been widely used, particularly in postoperative analgesia in children and cesarean section. However, the effect of oxycodone on developing brain still remains to be seen. Since there is a link between anesthetics exposure and long-term behavioral or cognitive dysfunction in young children, in the current study, the direct effect of oxycodone on neural stem cells (NSCs) biological behaviors was investigated. After exposed to a high dose of oxycodone (10 μg/mL) for 48 h, NSCs survival and proliferation were significantly reduced, while NSCs apoptosis and differentiation were enhanced. These effects were significantly weaker than that when exposed to same dose of morphine. No significant difference was observed regarding to above biological behaviors when exposed to lower doses (0.1 μg/mL and 1.0 μg/mL) of oxycodone. The antagonist of opioid receptor, nalmefene, successfully reversed the influence of oxycodone. Taken together, our results indicated that short term exposure to oxycodone in low dose might be allowed for developing brain.

Increase in Blood Levels of Growth Factors Involved in the Neuroplasticity Process by Using an Extremely Low Frequency Electromagnetic Field in Post-stroke Patients

Background: Neuroplasticity ensures the improvement of functional status in patients after stroke. The aim of this study was to evaluate the effect of extremely low-frequency electromagnetic field therapy (ELF-EMF) on brain plasticity in the rehabilitation of patients after stroke.

Methods: Forty-eight patients were divided into two groups underwent the same rehabilitation program, but in the study group, the patients additionally were exposed to a standard series of 10 ELF-EMF treatments. To determine the level of neuroplasticity, we measured the plasma level of the brain-derived neurotrophic factor (BDNF), the vascular-endothelial growth factor, as well as BDNF mRNA expression. Additionally, we determined the molecule levels for hepatocyte growth factor, stem cell factor, stromal cell-derived factor 1α, nerve growth factor β, and leukemia inhibitory factor, using 5plex cytokine panel in plasma. After 4 weeks, during which patients had undergone neurorehabilitation and neurological examinations, we assessed functional recovery using the Barthel Index, Mini-Mental State Examination (MMSE), Geriatric Depression Scale, National Institutes of Health Stroke Scale (NIHSS), and the modified Rankin Scale (mRS).

Results: We observed that ELF-EMF significantly increased growth factors and cytokine levels involved in neuroplasticity, as well as promoted an enhancement of functional recovery in post-stroke patients. Additionally, we presented evidence that these effects could be related to the increase of gene expression on the mRNA level. Moreover, a change of BDNF plasma level was positively correlated with the Barthel Index, MMSE, and negatively correlated with GDS.

Conclusion: Extremely low-frequency electromagnetic field therapy improves the effectiveness of rehabilitation of post-stroke patients by improving neuroplasticity processes.

Neurotherapeutic capacity of P7C3 agents for the treatment of Traumatic Brain Injury

Traumatic brain injury (TBI) is a significant public health problem around the world. A promising area of research is the characterization of small, drug-like molecules that have potent clinical properties. One pharmacotherapeutic agent in particular, an aminopropyl carbazole called P7C3, was discovered using an in vivo screen to identify new agents that augmented the net magnitude of adult hippocampal neurogenesis. P7C3 greatly enhanced neurogenesis by virtue of increasing survival rates of immature neurons. The potent neuroprotective efficacy of P7C3 is likely due to enhanced nicotinamide phosphoribosyltransferase (NAMPT) activity, which supports critical cellular processes. The scaffold of P7C3 was found to have favorable pharmacokinetic properties, good bioavailability, and was nontoxic. Preclinical studies have shown that administration of the P7C3-series of neuroprotective compounds after TBI can rescue and reverse detrimental cellular events leading to improved functional recovery. In several TBI models and across multiple species, P7C3 and its analogues have produced significant neuroprotection, axonal preservation, robust increases in the net magnitude of adult neurogenesis, protection from injury-induced LTP deficits, and improvement in neurological functioning. This review will elucidate the exciting and diverse therapeutic findings of P7C3 administration in the presence of a complex and multifactorial set of cellular and molecular challenges brought forth by experimental TBI. The clinical potential and broad therapeutic applicability of P7C3 warrants much needed investigation into whether these remedial effects can be replicated in the clinic. P7C3 may serve as an important step forward in the design, understanding, and implementation of pharmacotherapies for treating patients with TBI. This article is part of the Special Issue entitled "Novel Treatments for Traumatic Brain Injury".

Hippocampal granule cell loss in human chronic alcohol abusers

Chronic alcohol abuse causes cognitive impairments associated with neurodegeneration and volume loss in the human hippocampus. Here, we hypothesize that alcohol reduces the number of granule cells in the human dentate gyrus and consequently contribute to the observed volume loss. Hippocampal samples were isolated from deceased donors with a history of chronic alcohol abuse and from controls with no alcohol overconsumption. From each case, a sample from the mid-portion of hippocampus was sectioned, immunostained for the neuronal nuclear marker NeuN, and counter stained with hematoxylin. Granule cell number and volume of granular cell layer in the dentate gyrus were estimated using stereology. We found a substantial reduction in granule cell number and also a significantly reduced volume of the granular cell layer of chronic alcohol abusers as compared to controls. In controls there was a slight age-related decline in the number of granule cells and volume of granular cell layer in line with previous studies. This was not observed among the alcoholics, possibly due to a larger impact of alcohol abuse than age on the degenerative changes in the dentate gyrus. Loss of neurons in the alcoholic group could either be explained by an increase of cell death or a reduced number of new cells added to the granular cell layer. However, there is no firm evidence for an increased neuronal death by chronic alcohol exposure, whereas a growing body of experimental data indicates that neurogenesis is impaired by alcohol. In a recent study, we reported that alcoholics show a reduced number of stem/progenitor cells and immature neurons in the dentate gyrus, hence that alcohol negatively affects hippocampal neurogenesis. The present results further suggest that such impairment of neurogenesis by chronic alcohol abuse also results in a net loss of granule cells in the dentate gyrus of hippocampus.

Morphine regulates adult neurogenesis and contextual memory extinction via the PKCε/Prox1 pathway

We have previously reported that the miR-181a/Prox1/Notch1 pathway mediates the effect of morphine on modulating lineage-specific differentiation of adult neural stem/progenitor cells (NSPCs) via a PKCε-dependent pathway, whereas fentanyl shows no such effect. However, the role of the PKCε/Prox1 pathway in mediating drug-associated contextual memory remains unknown. The current study investigated the effect of PKCε/Prox1 on morphine-induced inhibition of adult neurogenesis and drug-associated contextual memory in mice, while the effect of fentanyl was tested simultaneously. By using BrdU labeling, we were able to examine the lineages of differentiated NSPCs in adult DG. PKCε knockout blocked morphine's effects on inducing in vivo astrocyte-preferential differentiation of NSPCs, but did not alter NSPC lineages upon fentanyl treatment. Inhibited adult neurogenesis further resulted in prolonged extinction and enhanced reinstatement of morphine-induced CPP, as well as prolonged extinction of space reference memory indicated by the Morris water maze paradigm. However, after fentanyl administration, no significant changes were found between wild-type and PKCε knockout mice, during either CPP or water maze tasks. When the lentivirus encoding Nestin-promoter-controlled Prox1 cDNA was injected into hippocampi of wildtype and PKCε knockout adult mice to modulate PKCε/Prox1 activity, similar effects were discovered in adult mice injected with lentivirus encoding Prox1, and more dramatic effects were found in PKCε knockout mice with concurrent Prox1 overexpression. In conclusion, morphine mediates lineage-specific NSPC differentiation, inhibits adult neurogenesis and regulates contextual memory retention via the PKCε/Prox1 pathway, which are implicated in the eventual context-associated relapse.

AhR Deletion Promotes Aberrant Morphogenesis and Synaptic Activity of Adult-Generated Granule Neurons and Impairs Hippocampus-Dependent Memory

Newborn granule cells are continuously produced in the subgranular zone of dentate gyrus throughout life. Once these cells mature, they integrate into pre-existing circuits modulating hippocampus-dependent memory. Subsequently, mechanisms controlling generation and maturation of newborn cells are essential for proper hippocampal function. Therefore, we have studied the role of aryl hydrocarbon receptor (AhR), a ligand-activated bHLH-PAS transcription factor, in hippocampus-dependent memory and granule neuronal morphology and function using genetic loss-of-function approaches based on constitutive and inducible-nestin AhR^–/– mice. The results presented here show that the impaired hippocampus-dependent memory in AhR absence is not due to its effects on neurogenesis but to aberrant dendritic arborization and an increased spine density, albeit with a lower number of mature mushrooms spines in newborn granule cells, a finding that is associated with an immature electrophysiological phenotype. Together, our data strongly suggest that AhR plays a pivotal role in the regulation of hippocampal function, by controlling hippocampal granule neuron morphology and synaptic maturation.

Inhibitory Effects of Auraptene and Naringin on Astroglial Activation, Tau Hyperphosphorylation, and Suppression of Neurogenesis in the Hippocampus of Streptozotocin-Induced Hyperglycemic Mice

Auraptene, a citrus-related compound, exerts anti-inflammatory effects in peripheral tissues, and we demonstrated these effects in the brains of a lipopolysaccharide-injected systemic inflammation animal model and a brain ischemic mouse model. Naringin, another citrus-related compound, has been shown to exert antioxidant effects in several animal models. Hyperglycemia induces oxidative stress and inflammation and causes extensive damage in the brain; therefore, we herein evaluated the anti-inflammatory and other effects of auraptene and naringin in streptozotocin-induced hyperglycemic mice. Both compounds inhibited astroglial activation and the hyperphosphorylation of tau at 231 of threonine in neurons, and also recovered the suppression of neurogenesis in the dentate gyrus of the hippocampus in hyperglycemic mice. These results suggested that auraptene and naringin have potential effects as neuroprotective agents in the brain.

BACE1 Regulates Proliferation and Neuronal Differentiation of Newborn Cells in the Adult Hippocampus in Mice

β-Site amyloid precursor protein cleaving enzyme 1 (BACE1) is required for the production of β-amyloid (Aβ), one of the major pathogenic molecules of Alzheimer's disease (AD), and is therefore being actively pursued as a drug target for AD. Adult hippocampal neurogenesis (AHN) is a lifelong process that is known to be important for learning and memory and may have the potential to regenerate damaged neural tissue. In this study, we examined whether BACE1 regulates AHN, which holds important implications for its suitability as a drug target in AD. Cohorts of 2-month-old wild-type (BACE1^+/+), heterozygous, and homozygous BACE1 knockout mice (BACE1^+/- and BACE1^-/-, respectively) were injected with 5-bromo-2'-deoxyuridine (BrdU) and sacrificed 1 day later to examine the impact of loss of BACE1 on neural precursor cell (NPC) proliferation in the adult brain. Parallel cohorts of mice were sacrificed 4 weeks after BrdU injection to determine the effects of BACE1 on survival and differentiation of newborn NPCs. We found that NPC proliferation was increased in BACE1^-/- mice compared to BACE1^+/+ mice, while no difference was observed in NPC survival across genotypes. Differentiation of NPCs to neuronal lineage was impaired in BACE1^-/- mice. However, no differences were observed in astrogenesis, the proportion of immature neurons, or the production of oligodendrocytes across genotypes. Importantly, corresponding with a decrease in neuronal differentiation in the absence of a complementary increase in an alternate cell fate, BACE1^-/- mice were found to have a pool of undifferentiated NPCs in the hippocampus compared to BACE1^+/+ and BACE1^+/- mice.

Suppressive effects of the peel of Citrus kawachiensis (Kawachi Bankan) on astroglial activation, tau phosphorylation, and inhibition of neurogenesis in the hippocampus of type 2 diabetic db/db mice

We previously reported that the dried peel powder of Citrus kawachiensis exerted anti-inflammatory effects in the brain in several animal models. Hyperglycemia induces inflammation and oxidative stress and causes massive damage in the brain; therefore, we herein examined the anti-inflammatory and other effects of the dried peel powder of C. kawachiensis in the streptozotocin-induced hyperglycemia mice model and in the type 2 diabetic db/db mice model. The C. kawachiensis administration inhibited microglial activation in the hippocampus in the streptozotocin-injected mice. Moreover, The C. kawachiensis treatment inhibited astroglial activation in the hippocampus and the hyperphosphorylation of tau at 231 of threonine and 396 of serine in hippocampal neurons, and also relieved the suppression of neurogenesis in the dentate gyrus of the hippocampus in the db/db mice. It was suggested that the dried peel powder of C. kawachiensis exerts anti-inflammatory and neuroprotective effects in the brain.

Adolescent Isolation Interacts With DISC1 Point Mutation to Impair Adult Social Memory and Synaptic Functions in the Hippocampus

Disrupted-in-schizophrenia 1 (DISC1) is a strong candidate susceptibility gene for a spectrum of neuropsychiatric diseases including schizophrenia, bipolar disorder and major depression, all of which are thought to result from interactions between gene mutations and environmental risk factors such as influenza, trauma and stress. Adolescence is a key period susceptible to stress and stress-related mental illnesses. In a previous study, we found that although DISC1 L100P point mutation mice shows object recognition deficits, their sociability and social memory are relatively normal. Therefore, in this article, we investigated whether the interaction between adolescent stress and DISC1 L100P point mutation affects adult social memory, and we explored the underlying mechanisms. We found that adolescent stress (isolation from 5 weeks to 8 weeks of age) specifically impaired social memory of adult DISC1 L100P mice but not that of WT littermates, which could be rescued by administration of atypical antipsychotic drug clozapine. On the other hand, it did not induce anxiety or depression in adult mice. Adolescent isolation exacerbated adult neurogenesis deficits in the hippocampus of DISC1 L100P mice, while it had no effect on WT mice. In addition, we found that adolescent isolation led to long lasting changes in synaptic transmission and plasticity in the hippocampal circuits, some of which are specific for DISC1 L100P mice. In summary, we identified here the specific interaction between genetic mutation (DISC1 L100P) and adolescence social stress that damages synaptic function and social memory in adult hippocampal circuits.

Highlights–

Adolescent isolation (from 5 weeks to 8 weeks of age) impairs adult social memory when combined with DISC1 L100P point mutation.

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Adolescent isolation exacerbates adult neurogenesis deficit in the hippocampus of L100P mice but has no similar effect on WT mice.

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Adolescent isolation causes long lasting changes in synaptic transmission and plasticity of the hippocampal network in DISC1 L100P mice.

BMP and WNT signalling cooperate through LEF1 in the neuronal specification of adult hippocampal neural stem and progenitor cells

Neuronal production from neural stem cells persists during adulthood in the subgranular zone of the hippocampal dentate gyrus. Extracellular signals provided by the hippocampal microenvironment regulate the neuronal fate commitment of the stem cell progeny. To date, the identity of those signals and their crosstalk has been only partially resolved. Here we show that adult rat hippocampal neural stem and progenitor cells (AH-NSPCs) express receptors for bone morphogenetic proteins (BMPs) and that the BMP/P-Smad pathway is active in AH-NSPCs undergoing differentiation towards the neuronal lineage. In vitro, exposure to the BMP2 and BMP4 ligands is sufficient to increase neurogenesis from AH-NSPCs in a WNT dependent manner while decreasing oligodendrogenesis. Moreover, BMP2/4 and WNT3A, a key regulator of adult hippocampal neurogenesis, cooperate to further enhance neuronal production. Our data point to a mechanistic convergence of the BMP and WNT pathways at the level of the T-cell factor/lymphoid enhancer factor gene Lef1. Altogether, we provide evidence that BMP signalling is an important regulator for the neuronal fate specification of AH-NSPCs cultures and we show that it significantly cooperates with the previously described master regulator of adult hippocampal neurogenesis, the WNT signalling pathway.

Neural stem cell heterogeneity in the mammalian forebrain

The brain was long considered an organ that underwent very little change after development. It is now well established that the mammalian central nervous system contains neural stem cells that generate progeny that are capable of making new neurons, astrocytes, and oligodendrocytes throughout life. The field has advanced rapidly as it strives to understand the basic biology of these precursor cells, and explore their potential to promote brain repair. The purpose of this review is to present current knowledge about the diversity of neural stem cells in vitro and in vivo, and highlight distinctions between neural stem cell populations, throughout development, and within the niche. A comprehensive understanding of neural stem cell heterogeneity will provide insights into the cellular and molecular regulation of neural development and lifelong neurogenesis, and will guide the development of novel strategies to promote regeneration and neural repair.

The role of neuro-epithelial-like and radial-glial stem and progenitor cells in development, plasticity, and repair

Neural stem and progenitor cells (NSPCs) are the primary source of new neurons in the brain and serve critical roles in tissue homeostasis and plasticity throughout life. Within the vertebrate brain, NSPCs are located within distinct neurogenic niches differing in their location, cellular composition, and proliferative behaviour. Heterogeneity in the NSPC population is hypothesized to reflect varying capacities for neurogenesis, plasticity and repair between different neurogenic zones. Since the discovery of adult neurogenesis, studies have predominantly focused on the behaviour and biological significance of adult NSPCs (aNSPCs) in rodents. However, compared to rodents, who show lifelong neurogenesis in only two restricted neurogenic niches, zebrafish exhibit constitutive neurogenesis across multiple stem cell niches that provide new neurons to every major brain division. Accordingly, zebrafish are a powerful model to probe the unique cellular and molecular profiles of NSPCs and investigate how these profiles govern tissue homeostasis and regenerative plasticity within distinct stem cell populations over time. Amongst the NSPC populations residing in the zebrafish central nervous system (CNS), proliferating radial-glia, quiescent radial-glia and neuro-epithelial-like cells comprise the majority. Here, we provide insight into the extent to which these distinct NSPC populations function and mature during development, respond to experience, and contribute to successful CNS regeneration in teleost fish. Together, our review brings to light the dynamic biological roles of these individual NSPC populations and showcases their diverse regenerative modes to achieve vertebrate brain repair later in life.

Development of Microplatforms to Mimic the In Vivo Architecture of CNS and PNS Physiology and Their Diseases

Understanding the mechanisms that govern nervous tissues function remains a challenge. In vitro two-dimensional (2D) cell culture systems provide a simplistic platform to evaluate systematic investigations but often result in unreliable responses that cannot be translated to pathophysiological settings. Recently, microplatforms have emerged to provide a better approximation of the in vivo scenario with better control over the microenvironment, stimuli and structure. Advances in biomaterials enable the construction of three-dimensional (3D) scaffolds, which combined with microfabrication, allow enhanced biomimicry through precise control of the architecture, cell positioning, fluid flows and electrochemical stimuli. This manuscript reviews, compares and contrasts advances in nervous tissues-on-a-chip models and their applications in neural physiology and disease. Microplatforms used for neuro-glia interactions, neuromuscular junctions (NMJs), blood-brain barrier (BBB) and studies on brain cancer, metastasis and neurodegenerative diseases are addressed. Finally, we highlight challenges that can be addressed with interdisciplinary efforts to achieve a higher degree of biomimicry. Nervous tissue microplatforms provide a powerful tool that is destined to provide a better understanding of neural health and disease.

Adult Hippocampal Neurogenesis: Regulation and Possible Functional and Clinical Correlates

The formation of new neurons in the adult central nervous system (CNS) has been recognized as one of the major findings in neuroanatomical research. The hippocampal formation (HF), one of the main targets of these investigations, holds a neurogenic niche widely recognized among several mammalian species and whose existence in the human brain has sparked controversy and extensive debate. Many cellular features from this region emphasize that hippocampal neurogenesis suffers changes with normal aging and, among regulatory factors, physical exercise and chronic stress provoke opposite effects on cell proliferation, maturation and survival. Considering the numerous functions attributable to the HF, increasing or decreasing the integration of new neurons in the delicate neuronal network might be significant for modulation of cognition and emotion. The role that immature and mature adult-born neurons play in this circuitry is still mostly unknown but it could prove fundamental to understand hippocampal-dependent cognitive processes, the pathophysiology of depression, and the therapeutic effects of antidepressant medication in modulating behavior and mental health.

Dopaminergic dysregulation and impaired associative learning behavior in zebrafish during chronic dietary exposure to selenium

A growing body of evidence indicates that exposure to selenium (Se) can cause neurotoxicity, and this can occur because of its interference with several neurotransmitter systems in humans and animals. Dopamine is a critical modulator of a variety of brain functions and a prime target for environmental neurotoxicants. However, effects of environmentally relevant concentrations of Se on dopaminergic system and its neurobehavioral effects are still largely unknown. For this purpose, we exposed zebrafish, a model organism, to different concentrations of dietary l-selenomethionine (control, 3.5, 11.1, 27.4, and 63.4 μg Se/g dry weight) for a period of 60 days. Cognitive performance of fish was evaluated using a plus maze associative learning paradigm. Oxidative stress, as the main driver of Se neurotoxicity, was assessed by measuring the ratio of reduced to oxidized glutathione (GSH:GSSG), lipid peroxidation (LPO) levels, and mRNA expression of several antioxidant enzymes in the zebrafish brain. Dopamine levels in the brain and the expression of genes involved in dopamine synthesis, storage, reuptake, metabolism, and receptor activation were examined. Moreover, transcription of several synaptic plasticity-related immediate-early and late response genes was determined. Overall, fish fed with the two highest concentrations of dietary Se displayed impaired associative learning. Se exposure also induced oxidative stress in the zebrafish brain, as indicated by a reduction in GSH:GSSG ratio, increased LPO levels, and up-regulation of antioxidant genes in fish treated with the two highest concentrations of Se. An increase in brain dopamine levels associated with altered expression of dopaminergic cell markers was evident in different treatment groups. Moreover, Se exposure led to the down-regulation of immediate-early and late response genes in fish that exhibiting learning impairment. Taken together, the results of this study imply that the induction of oxidative stress and dysregulation of dopaminergic neurotransmission may underlie Se-induced impairment of associative learning in zebrafish.

Neurogenesis: remembering all or forgetting some

Research over the last 20 years has begun to elucidate the importance of adult neurogenesis in cognition. Three studies recently asked what might be happening to memories formed before neurogenesis in the hippocampus (Akers KG, Martinez-Canabal A, Restivo L, Yiu AP, De Cristofaro A, Hsiang HL, Wheeler AL, Guskjolen A, Niibori Y, Shoji H, Ohira K, Richards BA, Miyakawa T, Josselyn SA, Frankland PW. Science 344: 598–602, 2014; Epp JR, Silva Mera R, Köhler S, Josselyn SA, Frankland PW. Nat Commun 7: 10838, 2016; Kodali M, Megahed T, Mishra V, Shuai B, Hattiangady B, Shetty AK. J Neurosci 36: 8112–8122, 2016). These studies found conflicting results: running (which increases neurogenesis) induced forgetting in two studies, but there was no difference in memory retrieval after exercise in another. To reconcile these studies, one must understand the processes behind memory maintenance and recall and consider context, species, and other factors.

Disruption of the Microglial ADP Receptor P2Y13 Enhances Adult Hippocampal Neurogenesis

In mammalian species, including humans, the hippocampal dentate gyrus (DG) is a primary region of adult neurogenesis. Aberrant adult hippocampal neurogenesis is associated with neurological pathologies. Understanding the cellular mechanisms controlling adult hippocampal neurogenesis is expected to open new therapeutic strategies for mental disorders. Microglia is intimately associated with neural progenitor cells in the hippocampal DG and has been implicated, under varying experimental conditions, in the control of the proliferation, differentiation and survival of neural precursor cells. But the underlying mechanisms remain poorly defined. Using fluorescent in situ hybridization we show that microglia in brain express the ADP-activated P2Y₁₃ receptor under basal conditions and that P2ry13 mRNA is absent from neurons, astrocytes, and neural progenitor cells. Disrupting P2ry13 decreases structural complexity of microglia in the hippocampal subgranular zone (SGZ). But it increases progenitor cell proliferation and new neuron formation. Our data suggest that P2Y₁₃ receptor-activated microglia constitutively attenuate hippocampal neurogenesis. This identifies a signaling pathway whereby microglia, via a nucleotide-mediated mechanism, contribute to the homeostatic control of adult hippocampal neurogenesis. Selective P2Y₁₃R antagonists could boost neurogenesis in pathological conditions associated with impaired hippocampal neurogenesis.

Gender-dependent changes in physical development, BDNF content and GSH redox system in a model of acute neonatal hypoxia in rats

Perinatal hypoxia-ischaemia is one of the leading factors that negatively influence the development of the central nervous system. Our aim was to investigate the effects of sex on the outcomes of acute neonatal hypoxia (ANH) in rat pups. Male and female Wistar rats were exposed to a hypoxic condition (8% oxygen for 120 min) at postnatal day 2 (P2). Immediately after ANH an increase in HIF1-α gene expression was observed in the rat brains, independently of sex. Brain-derived neurotrophic factor (BDNF) and glutathione peroxidase-4 gene expression was increased in female animals only. Hypoxic pups of both sexes showed a decreased reduced/oxidised glutathione (GSH/GSSG) ratio in the blood and only males had an increased GSH content in the whole brain immediately after hypoxia. Furthermore, an increased BDNF content in the brain was found in both male and female rat pups at 0 h and in serum 4 h after hypoxia, but at 4 h after hypoxia only males had an increased BDNF level in the brain. Only hypoxic males displayed retarded performance in the righting reflex, but in a negative geotaxis test hypoxic pups of both sexes had an increased turnaround time. Moreover, hypoxic female but not male pups demonstrated less weight gain than control littermates for the entire observation period (until P18). These results demonstrate that ANH at P2 leads to both molecular and physiological impairments in a sex-specific manner and the described model could be used to represent mild hypoxic brain damage in very preterm infants.

Adult neurogenesis in the hippocampus from a perspective of discrimination and generalization: a hypothesis

The function of adult neurogenesis in the dentate gyrus is not yet completely understood, though many competing theories have attempted to explain the function of these newly-generated neurons. Most theories give adult neurogenesis a role in aiding known hippocampal/dentate gyrus functions. Other theories offer a novel role for these new cells based on their unique physiological qualities, such as their low excitability threshold. Many behavioral tests have been used to test these theories, but results have been inconsistent and often contradictory. Substantial variability in tests and protocols may be at least partially responsible for the mixed results. On the other hand, conflicting results arising from the same tests can serve as aids in elucidating the function of adult neurogenesis. Here, we offer a hypothesis that considers the cognitive nature of tasks commonly used to assess the function of adult neurogenesis, and introduce a dichotomy between tasks focused on discrimination vs. generalization. We view these two aspects as opposite ends of the continuous spectrum onto which traditional tests can be mapped. We propose that high neurogenesis favors behavioral discrimination while low adult neurogenesis favors behavioral generalization of a knowledge or rule. Since many tasks require both, the effects of neurogenesis could be cancelled out in many cases. Although speculative, we hope that our view presents an interesting and testable hypothesis of the effect of adult neurogenesis in traditional behavioral tasks. We conclude that new, carefully designed behavioral tests may be necessary to reach a final consensus on the role of adult neurogenesis in behavior.

RNA-Sequencing Reveals Unique Transcriptional Signatures of Running and Running-Independent Environmental Enrichment in the Adult Mouse Dentate Gyrus

Environmental enrichment (EE) is a powerful stimulus of brain plasticity and is among the most accessible treatment options for brain disease. In rodents, EE is modeled using multi-factorial environments that include running, social interactions, and/or complex surroundings. Here, we show that running and running-independent EE differentially affect the hippocampal dentate gyrus (DG), a brain region critical for learning and memory. Outbred male CD1 mice housed individually with a voluntary running disk showed improved spatial memory in the radial arm maze compared to individually- or socially-housed mice with a locked disk. We therefore used RNA sequencing to perform an unbiased interrogation of DG gene expression in mice exposed to either a voluntary running disk (RUN), a locked disk (LD), or a locked disk plus social enrichment and tunnels [i.e., a running-independent complex environment (CE)]. RNA sequencing revealed that RUN and CE mice showed distinct, non-overlapping patterns of transcriptomic changes versus the LD control. Bio-informatics uncovered that the RUN and CE environments modulate separate transcriptional networks, biological processes, cellular compartments and molecular pathways, with RUN preferentially regulating synaptic and growth-related pathways and CE altering extracellular matrix-related functions. Within the RUN group, high-distance runners also showed selective stress pathway alterations that correlated with a drastic decline in overall transcriptional changes, suggesting that excess running causes a stress-induced suppression of running’s genetic effects. Our findings reveal stimulus-dependent transcriptional signatures of EE on the DG, and provide a resource for generating unbiased, data-driven hypotheses for novel mediators of EE-induced cognitive changes.

Liver X receptor β in the hippocampus: A potential novel target for the treatment of major depressive disorder?

Liver X receptors (LXRs), including LXRα and LXRβ isoforms, have been implicated in multiple physiological functions including promoting neurogenesis, improving synaptic plasticity, preventing neurodegeneration, inhibiting inflammation as well as regulating cholesterol metabolism. However, a potential role of LXRs in the treatment of major depressive disorder (MDD) has never been investigated previously. Our present results demonstrated that levels of hippocampal LXRβ but not LXRα were down-regulated in rats exposed to chronic unpredictable stress (CUS) and were negatively correlated with the severity of CUS-induced depressive-like behaviors. Furthermore, rats with LXRβ knockdown by short hairpin RNA (shRNA) in hippocampus displayed depressive-like behaviors and impaired hippocampal neurogenesis similar to those observed after CUS exposure. Conversely, LXRs activation by GW3965 (GW), a synthetic dual agonist for both LXRα and LXRβ isoforms, could improve depression-like behaviors and reverse the impaired hippocampal neurogenesis in rats exposed to CUS. LXRβ knockdown by shRNA completely abrogated the antidepressant and hippocampal neurogenesis-promoting effects of GW, suggesting that LXRβ isoform mediated the antidepressant and hippocampal neurogenesis-promoting effects of the LXRα/β dual agonist. However, ablation of hippocampal neurogenesis with x-irradiation only partly but not completely abolished the antidepressant effects of GW in the behavioral tests, implying that the antidepressant effects mediated by LXRβ isoform are likely through both neurogenesis-dependent and -independent pathways. Thus, our findings suggest that LXRβ activation may represent a potential novel target for the treatment of MDD and also provide a novel insight into the underlying mechanisms of MDD.

Selective alteration of adult hippocampal neurogenesis and impaired spatial pattern separation performance in the RSK2-deficient mouse model of Coffin-Lowry syndrome

Adult neurogenesis is involved in certain hippocampus-dependent cognitive functions and is linked to psychiatric diseases including intellectual disabilities. The Coffin-Lowry syndrome (CLS) is a developmental disorder caused by mutations in the Rsk2 gene and characterized by intellectual disabilities associated with growth retardation. How RSK2-deficiency leads to cognitive dysfunctions in CLS is however poorly understood. Here, using Rsk2 Knock-Out mice, we characterized the impact of RSK2 deficiency on adult hippocampal neurogenesis in vivo. We report that the absence of RSK2 does not affect basal proliferation, differentiation and survival of dentate gyrus adult-born neurons but alters the maturation progression of young immature newborn neurons. Moreover, when RSK2-deficient mice were submitted to spatial learning, in contrast to wild-type mice, proliferation of adult generated neurons was decreased and no pro-survival effect of learning was observed. Thus, learning failed to recruit a selective population of young newborn neurons in association with deficient long-term memory recall. Given the proposed role of the dentate gyrus and of adult-generated newborn neurons in hippocampal-dependent pattern separation function, we explored this function in a delayed non-matching to place task and in an object-place pattern separation task and report severe deficits in spatial pattern separation in Rsk2-KO mice. Together, this study reveals a previously unknown role for RSK2 in the early stages of maturation and learning-dependent involvement of adult-born dentate gyrus neurons. These alterations associated with a deficit in the ability of RSK2-deficient mice to finely discriminate relatively similar spatial configurations, may contribute to cognitive dysfunction in CLS.

Fluoxetine attenuates the impairment of spatial learning ability and prevents neuron loss in middle-aged APPswe/PSEN1dE9 double transgenic Alzheimer's disease mice

Selective serotonin reuptake inhibitors (SSRIs) have been reported to increase cognitive performance in some clinical studies of Alzheimer's disease (AD). However, there is a lack of evidence supporting the efficacy of SSRIs as cognition enhancers in AD, and the role of SSRIs as a treatment for AD remains largely unclear. Here, we characterized the impact of fluoxetine (FLX), a well-known SSRI, on neurons in the dentate gyrus (DG) and in CA1 and CA3 of the hippocampus of middle-aged (16 to 17 months old) APPswe/PSEN1dE9 (APP/PS1) transgenic AD model mice. We found that intraperitoneal (i.p.) injection of FLX (10 mg/kg/day) for 5 weeks effectively alleviated the impairment of spatial learning ability in middle-aged APP/PS1 mice as evaluated using the Morris water maze. More importantly, the number of neurons in the hippocampal DG was significantly increased by FLX. Additionally, FLX reduced the deposition of beta amyloid, inhibited GSK-3β activity and increased the level of β-catenin in middle-aged APP/PS1 mice. Collectively, the results of this study indicate that FLX delayed the progression of neuronal loss in the hippocampal DG in middle-aged AD mice, and this effect may underlie the FLX-induced improvement in learning ability. FLX may therefore serve as a promising therapeutic drug for AD.

BDNF: A Key Factor with Multipotent Impact on Brain Signaling and Synaptic Plasticity

Brain-derived neurotrophic factor (BDNF) is one of the most widely distributed and extensively studied neurotrophins in the mammalian brain. Among its prominent functions, one can mention control of neuronal and glial development, neuroprotection, and modulation of both short- and long-lasting synaptic interactions, which are critical for cognition and memory. A wide spectrum of processes are controlled by BDNF, and the sometimes contradictory effects of its action can be explained based on its specific pattern of synthesis, comprising several intermediate biologically active isoforms that bind to different types of receptor, triggering several signaling pathways. The functions of BDNF must be discussed in close relation to the stage of brain development, the different cellular components of nervous tissue, as well as the molecular mechanisms of signal transduction activated under physiological and pathological conditions. In this review, we briefly summarize the current state of knowledge regarding the impact of BDNF on regulation of neurophysiological processes. The importance of BDNF for future studies aimed at disclosing mechanisms of activation of signaling pathways, neuro- and gliogenesis, as well as synaptic plasticity is highlighted.

Impaired neuronal maturation of hippocampal neural progenitor cells in mice lacking CRAF

RAF kinases are major constituents of the mitogen activated signaling pathway, regulating cell proliferation, differentiation and cell survival of many cell types, including neurons. In mammals, the family of RAF proteins consists of three members, ARAF, BRAF, and CRAF. Ablation of CRAF kinase in inbred mouse strains causes major developmental defects during fetal growth and embryonic or perinatal lethality. Heterozygous germline mutations in CRAF result in Noonan syndrome, which is characterized by neurocognitive impairment that may involve hippocampal physiology. The role of CRAF signaling during hippocampal development and generation of new postnatal hippocampal granule neurons has not been examined and may provide novel insight into the cause of hippocampal dysfunction in Noonan syndrome. In this study, by crossing CRAF-deficiency to CD-1 outbred mice, a CRAF mouse model was established which enabled us to investigate the interplay of neural progenitor proliferation and postmitotic differentiation during adult neurogenesis in the hippocampus. Albeit the general morphology of the hippocampus was unchanged, CRAF-deficient mice displayed smaller granule cell layer (GCL) volume at postnatal day 30 (P30). In CRAF-deficient mice a substantial number of abnormal, chromophilic, fast dividing cells were found in the subgranular zone (SGZ) and hilus of the dentate gyrus (DG), indicating that CRAF signaling contributes to hippocampal neural progenitor proliferation. CRAF-deficient neural progenitor cells showed an increased cell death rate and reduced neuronal maturation. These results indicate that CRAF function affects postmitotic neural cell differentiation and points to a critical role of CRAF-dependent growth factor signaling pathway in the postmitotic development of adult-born neurons.

Peptide Sharing Between Viruses and DLX Proteins: A Potential Cross-Reactivity Pathway to Neuropsychiatric Disorders

The present study seeks to determine potential associations between viral infections and neuropsychiatric diseases. To address this issue, we investigated the peptide commonalities between viruses that have been related to psychiatric and neurological disorders—such as rubella, human immunodeficiency virus, and herpesviruses—and human distal-less homeobox (DLX) proteins expressed in developing brain—namely, DLX1, DLX2, DLX5, and DLX6. Peptide matching analyses revealed a high degree of pentapeptide sharing. From an immunological perspective, this overlap is relevant because pentapeptides are endowed with immunogenicity and antigenicity—that is, they are immune determinants. Moreover, infection-induced immune cross-reactions might have functional, spatial, and temporal implications related to the functions and expression patterns of DLX1 and DLX5 in the fetal and adult human brain. In sum, our data support the hypothesis that viral infections may be linked to neuropsychiatric diseases through autoimmune cross-reactions caused by molecular mimicry between viral proteins and brain-specific DLX self-antigens.

Effects of physical activity on delayed memory measures in randomized controlled trials with nonclinical older, mild cognitive impairment, and dementia participants

INTRODUCTION

Longitudinal studies have found that physical activity protects against Alzheimer disease, but the mechanism is unknown. The prevailing model derives from animal research and has physical activity directly affecting brain physiology by increasing brain volume, brain-derived neurotrophic factor (BDNF), and hippocampal neurogenesis with consequent gains in neuropsychological test scores. Supporting evidence has been mixed, with physical-activity-related gains across multiple neuropsychological domains considered indicative of the protective effect. Hippocampal-mediated delayed memory functioning is the first neurocognitive skill to be impaired in the early stages of Alzheimer disease, and physical-activity-related gains on delayed memory measures would provide the strongest support for the model.

RESULTS

Review of 26 randomized controlled trials with nonclinical older, mild cognitive impairment, and dementia participants found only one with significant physical-activity-related gains in delayed memory compared to controls. This evidence does not support the physiological brain change model. Similarly, there is questionable support from those randomized controlled trials that have measured physical-activity-related brain volume and blood BDNF levels (neurogenesis having no valid labeling technique in living humans).

CONCLUSION

Physical-activity-related protective effects against Alzheimer disease are likely mediated through pathways outside the central nervous system.

Stress, Depression, Resilience and Ageing: A Role for the LPA-LPA1 Pathway

BACKGROUND

Chronic stress affects health and the quality of life, with its effects being particularly relevant in ageing due to the psychobiological characteristics of this population. However, while some people develop psychiatric disorders, especially depression, others seem very capable of dealing with adversity. There is no doubt that along with the identification of neurobiological mechanisms involved in developing depression, discovering which factors are involved in positive adaptation under circumstances of extreme difficulty will be crucial for promoting resilience.

METHODS

Here, we review recent work in our laboratory, using an animal model lacking the LPA1 receptor, together with pharmacological studies and clinical evidence for the possible participation of the LPA1 receptor in mood and resilience to stress.

RESULTS

Substantial evidence has shown that the LPA1 receptor is involved in emotional regulation and in coping responses to chronic stress, which, if dysfunctional, may induce vulnerability to stress and predisposition to the development of depression. Given that there is commonality of mechanisms between those involved in negative consequences of stress and in ageing, this is not surprising, considering that the LPA1 receptor may be involved in coping with adversity during ageing.

CONCLUSION

Alterations in this receptor may be a susceptibility factor for the presence of depression and cognitive deficits in the elderly population. However, because this is only a promising hypothesis based on previous data, future studies should focus on the involvement of the LPA-LPA1 pathway in coping with stress and resilience in ageing.

Decreased Hippocampal Neuroplasticity and Behavioral Impairment in an Animal Model of Inhalant Abuse

Thinners are highly toxic chemicals widely employed as organic solvents in industrial and domestic use. They have psychoactive properties when inhaled, and their chronic abuse as inhalants is associated with severe long-term health effects, including brain damage and cognitive-behavioral alterations. Yet, the sites and mechanisms of action of these compounds on the brain are far from being fully understood. Here, we investigated the consequences of paint thinner inhalation in adult male mice. Depression-like behaviors and an anxiolytic effect were found following repeated exposure in chronic treatments lasting 12 weeks. Both subchronic (6 weeks) and chronic treatments impaired learning and memory functions, while no changes were observed after acute treatment. To investigate possible molecular/structural alterations underlying such behavioral changes, we focused on the hippocampus. Notably, prolonged, but not acute thinner inhalation strongly affected adult neurogenesis in the dentate gyrus (DG), reducing progenitor cell proliferation after chronic treatments and impairing the survival of newborn neurons following both chronic and subchronic treatments. Furthermore, a down-regulation in the expression of BDNF and NMDA receptor subunits as well as a reduction in CREB expression/phosphorylation were found in the hippocampi of chronically treated mice. Our findings demonstrate for the first time significant structural and molecular changes in the adult hippocampus after prolonged paint thinner inhalation, indicating reduced hippocampal neuroplasticity and strongly supporting its implication in the behavioral dysfunctions associated to inhalant abuse.

Neurogenesis during Abstinence Is Necessary for Context-Driven Methamphetamine-Related Memory

Abstinence from methamphetamine addiction enhances proliferation and differentiation of neural progenitors and increases adult neurogenesis in the dentate gyrus (DG). We hypothesized that neurogenesis during abstinence contributes to context-driven drug-seeking behaviors. To test this hypothesis, the pharmacogenetic rat model (GFAP-TK rats) was used to conditionally and specifically ablate neurogenesis in the DG. Male GFAP-TK rats were trained to self-administer methamphetamine or sucrose and were administered the antiviral drug valganciclovir (Valcyte) to produce apoptosis of actively dividing GFAP type 1 stem-like cells to inhibit neurogenesis during abstinence. Hippocampus tissue was stained for Ki-67, NeuroD, and DCX to measure levels of neural progenitors and immature neurons, and was stained for synaptoporin to determine alterations in mossy fiber tracts. DG-enriched tissue punches were probed for CaMKII to measure alterations in plasticity-related proteins. Whole-cell patch-clamp recordings were performed in acute brain slices from methamphetamine naive (controls) and methamphetamine experienced animals (+/-Valcyte). Spontaneous EPSCs and intrinsic excitability were recorded from granule cell neurons (GCNs). Reinstatement of methamphetamine seeking enhanced autophosphorylation of CaMKII, reduced mossy fiber density, and induced hyperexcitability of GCNs. Inhibition of neurogenesis during abstinence prevented context-driven methamphetamine seeking, and these effects correlated with reduced autophosphorylation of CaMKII, increased mossy fiber density, and reduced the excitability of GCNs. Context-driven sucrose seeking was unaffected. Together, the loss-of-neurogenesis data demonstrate that neurogenesis during abstinence assists with methamphetamine context-driven memory in rats, and that neurogenesis during abstinence is essential for the expression of synaptic proteins and plasticity promoting context-driven drug memory.SIGNIFICANCE STATEMENT Our work uncovers a mechanistic relationship between neurogenesis in the dentate gyrus and drug seeking. We report that the suppression of excessive neurogenesis during abstinence from methamphetamine addiction by a confirmed phamacogenetic approach blocked context-driven methamphetamine reinstatement and prevented maladaptive changes in expression and activation of synaptic proteins and basal synaptic function associated with learning and memory in the dentate gyrus. Our study is the first to demonstrate an interesting and dysfunctional role of adult hippocampal neurogenesis during abstinence to drug-seeking behavior in animals self-administering escalating amounts of methamphetamine. Together, these results support a direct role for the importance of adult neurogenesis during abstinence in compulsive-like drug reinstatement.

Spatial geometry of stem cell proliferation in the adult hippocampus

The modes of stem cell divisions (e.g., symmetric vs. asymmetric) can have a profound impact on the number of progeny and tissue growth, repair, and function. This is particularly relevant for adult neural stem cells, since stem cell-derived neurons affect cognitive and mental states, resistance to stress and disease, and response to therapies. Here we show that although dividing stem cells in the adult hippocampus display a certain bias towards paired distribution (which could imply the prevalence of symmetric divisions), this bias already exists in the distribution of the general population of stem cells and may be responsible for the perceived occurrence of symmetric stem cell divisions. Remarkably, the bias in the distribution of stem cells decreases with age. Our results argue that the preexisting bias in stem cell distribution may affect current assumptions regarding stem cell division and fate as well as conjectures on the prospects of brain repair and rejuvenation.

Intrauterine inflammation reduces postnatal neurogenesis in the hippocampal subgranular zone and leads to accumulation of hilar ectopic granule cells

Prenatal inflammation is associated with poor neurobehavioral outcomes in exposed offspring. A common route of exposure for the fetus is intrauterine infection, which is often associated with preterm birth. Hippocampal development may be particularly vulnerable to an inflammatory insult during pregnancy as this region remains highly neurogenic both prenatally and postnatally. These studies sought to determine if intrauterine inflammation specifically altered hippocampal neurogenesis and migration of newly produced granule neurons during the early postnatal period. Microglial and astroglial cell populations known to play a role in the regulation of postnatal neurogenesis were also examined. We show that intrauterine inflammation significantly reduced hippocampal neurogenesis between postnatal days 7 (P7) and P14 as well as decreased granule cell density at P28. Ectopic migration of granule cells was observed in LPS-exposed mice at P14, but not at P28. Intrauterine inflammation had no effect on hippocampal astrocyte or microglia density or on apoptosis rate at the postnatal time points examined. Thus, exposure to intrauterine inflammation disrupts early postnatal neurogenesis and leads to aberrant migration of newly born granule cells.

The peel of Citrus kawachiensis (kawachi bankan) ameliorates microglial activation, tau hyper-phosphorylation, and suppression of neurogenesis in the hippocampus of senescence-accelerated mice

We previously reported that the dried peel powder of Citrus kawachiensis, one of the citrus products of Ehime, Japan, exerted anti-inflammatory effects in the brain of a lipopolysaccharide-injected systemic inflammation animal model. Inflammation is one of the main mechanisms underlying aging in the brain; therefore, we herein evaluated the anti-inflammatory and other effects of the dried peel powder of C. kawachiensis in the senescence-accelerated mouse-prone 8 (SAMP8) model. The C. kawachiensis treatment inhibited microglial activation in the hippocampus, the hyper-phosphorylation of tau at 231 of threonine in hippocampal neurons, and ameliorated the suppression of neurogenesis in the dentate gyrus of the hippocampus. These results suggest that the dried peel powder of C. kawachiensis exert anti-inflammatory and neuroprotective effects.

The Potential of Stem Cells in Treatment of Traumatic Brain Injury

PURPOSE OF REVIEW

Traumatic brain injury (TBI) is a global public health concern, with limited treatment options available. Despite improving survival rate after TBI, treatment is lacking for brain functional recovery and structural repair in clinic. Recent studies have suggested that the mature brain harbors neural stem cells which have regenerative capacity following brain insults. Much progress has been made in preclinical TBI model studies in understanding the behaviors, functions, and regulatory mechanisms of neural stem cells in the injured brain. Different strategies targeting these cell population have been assessed in TBI models. In parallel, cell transplantation strategy using a wide range of stem cells has been explored for TBI treatment in pre-clinical studies and some in clinical trials. This review summarized strategies which have been explored to enhance endogenous neural stem cell-mediated regeneration and recent development in cell transplantation studies for post-TBI brain repair.

RECENT FINDINGS

Thus far, neural regeneration through neural stem cells either by modulating endogenous neural stem cells or by stem cell transplantation has attracted much attention. It is highly speculated that targeting neural stem cells could be a potential strategy to repair and regenerate the injured brain. Neuroprotection and neuroregeneration are major aspects for TBI therapeutic development. With technique advancement, it is hoped that stem cell-based therapy targeting neuroregeneration will be able to translate to clinic in not so far future.

Impact of Swiprosin-1/Efhd2 on Adult Hippocampal Neurogenesis

Swiprosin-1/Efhd2 (Efhd2) is highly expressed in the CNS during development and in the adult. EFHD2 is regulated by Ca²⁺ binding, stabilizes F-actin, and promotes neurite extension. Previous studies indicated a dysregulation of EFHD2 in human Alzheimer's disease brains. We hypothesized a detrimental effect of genetic ablation of Efhd2 on hippocampal integrity and specifically investigated adult hippocampal neurogenesis. Efhd2 was expressed throughout adult neuronal development and in mature neurons. We observed a severe reduction of the survival of adult newborn neurons in Efhd2 knockouts, starting at the early neuroblast stage. Spine formation and dendrite growth of newborn neurons were compromised in full Efhd2 knockouts, but not upon cell-autonomous Efhd2 deletion. Together with our finding of severe hippocampal tauopathy in Efhd2 knockout mice, these data connect Efhd2 to impaired synaptic plasticity as present in Alzheimer's disease and identify a role of Efhd2 in neuronal survival and synaptic integration in the adult hippocampus.

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Efhd2 is expressed in the dentate gyrus and its loss reduces adult neurogenesis

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Reduced neurite complexity and spine density in new neurons of Efhd2 knockout mice

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Role of cell-extrinsic EFHD2 for dendrite morphology of adult newborn neurons

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Increased levels of pathological TAU in the hippocampus of Efhd2 knockout mice

The effect of combined therapies on recovery after acquired brain injury: Systematic review of preclinical studies combining enriched environment, exercise, or task-specific training with other therapies

BACKGROUND

Acquired brain injuries (ABI) have devastating effects for the affected individual as well as society. Many studies have investigated the effect of different monotherapies. However, functional recovery is typically only partial. One possible strategy to promote a greater degree of recovery is to apply monotherapies in combination with one or more treatments.

OBJECTIVE

The objective of this systematic review is to investigate if approaches combining enriched environment (EE), exercise, or task-specific training with other monotherapies, further enhance the degree of recovery after ABI.

METHOD

Scopus, PsychINFO, and PubMed databases were searched in March 2016 with the following search strings: exercise (or) enriched environment (or) environmental enrichment (or) rehabilitation (and) traumatic brain injury (or) ischemia (or) stroke (and) rat (or) rodent. Studies were included if they (1) were in English, (2) used adult animals subjected to brain injury, (3) included EE, and/or exercise, and/or task-specific training as post-injury treatment strategies, (4) included at least one group receiving another monotherapy. Out of 2.168 hits, 29 studies fulfilled the inclusion criteria.

RESULTS

Despite several trends for enhanced recovery after combined therapies, this systematic review of 29 studies does not indicate that combined therapies confer consistent combined effects on motor, cognitive, or cerebral recovery according to present criteria for combined effect.

CONCLUSION

Combined treatments continue to provide hope for enhanced recovery after ABI, however, the research area is in its infancy. This systematic review does not provide conclusive evidence. This is likely due to sparse knowledge regarding optimal treatment parameters. Combined treatments, however, hold the best promise regarding treatment of the complex changes induced by ABI.